Process for recovering sulfur and metal values from sulfur-bearing minerals

ABSTRACT

A process for recovering sulfur values and concentrating other metal values from pyrrhotite (FeS) by complexing ferrous sulfide in aqueous medium with an insoluble alkylamino polycarboxylic acid chelating agent to form a water-soluble ferrous chelate, hydrogen sulfide and a concentrate of insoluble sulfides of nonferrous metals. The hydrogen sulfide is reacted Claus sulfur dioxide under conventional Clause reaction conditions to yield sulfur. The ferrous chelate, in solution, is treated with an oxygen-free sulfur dioxide whereby the water-insoluble chelating agent is regenerated for recycle. The solution of ferrous bisulfite which forms is processed to yield additional quantities of sulfur dioxide from which sulfur is recovered.

United States Patent inventors Appl. No. Filed Patented Assignee PROCESSFOR RECOVERING SULFUR AND METAL VALUES FROM SULFUR-BEARING MINERALS 15Claims, 1 Drawing Fig.

11.8. C1 23/225, 23/130,23/181,75/l0l,260/429J Int. Cl C0lb 17/06 Fieldof Search ....23/224-225,

Primary Examiner Oscar R. Vertiz Assistant ExaminerG. O. PetersAttorneys-Gerard P. Rooney and Albert L. Gazzola ABSTRACT: A process forrecovering sulfur values and concentrating other metal values frompyrrhotite (FeS) by complexing ferrous sulfide in aqueous medium with aninsoluble alkylamino polycarboxylic acid chelating agent to form awater-soluble ferrous chelate, hydrogen sulfide and a concentrate ofinsoluble sulfides of nonferrous metals. The hydrogen sulfide is reactedClaus sulfur dioxide under conventional Clause reaction conditions toyield sulfur. The ferrous chelate, in solution, is treated with anoxygen-free sulfur dioxide whereby the water-insoluble chelating agentis regenerated for recycle. The solution of ferrous bisulfite whichforms is processed to yield additional quantities of sulfur dioxide fromwhich sulfur is recovered.

PATENTEDSEP21 9n PYRITE Fe 52 PYRRHOTITE s K r Fe 8. S O

I 2 I I ///UNDILUTED SMELTER ED A k I T 802 GAS OR OTHER R SOURCE OF soI Fe EDTA 2 I I 2 REE I 1 I I RECOVERED BOIL I EDTA Fe(HSO I Fe A I I lI I I I0 t I INVENTORS CHARLES P. BRUEN DONALD H. KELLY CHRISTIANA.WAMSER BHASKAR BANDYOPADHYAY PROCESS FOR RECOVERING SULFUR AND METALVALUES FROM SULFUR-BEARING MINERALS BACKGROUND OF THE INVENTION It isknown to recover sulfur by the process of roasting pyrite ore in a shaftfurnace or a fluidized bed reactor at temperatures of from 650-800 C.,in the presence of a reducing agent, wherein the sulfur dioxide producedunder this reducing atmosphere is converted to sulfur. This roastingprocess is used not only to recover sulfur, but also to recover metalvalues which may be present in pyrite ores. A process such as this isdisclosed in U.S. Pat. No. 2,869,999. The use of the roasting process toreduce sulfur from pyrite requires extremely high temperatures over longperiods and concomitantly high cost and operating difficulties. As aconsequence, for economic reasons pyrite, FeS,, has been decomposed bythermal desulfurization to recover only half the sulfur values. Aprocess such as this, in which pyrite is converted to pyrrhotite(essentially FeS) and sulfur, is an established process and isdocumented in detail in A Comprehensive Treatise in Inorganic Chemistry,J. W. Mellor, volume XIV, page 213.

The use of water insoluble alkylamino polycarboxylic acid chelatingagents for recovering sulfur from pyrrhotite was not known before thisinvention. It has been found that the ferrous iron in pyrrhotitecomplexes readily with the insoluble alkylamino polycarboxylic acidchelating agent in the process of the present invention, to formhydrogen sulfide, which is then converted to sulfur, and a solubleferrous complex from which the insoluble chelating agent may be readilyregenerated with sulfur dioxide for reuse. An added feature of thisinvention is that both the pyrite and pyrrhotite ores, and waste SOgases, which are used in the process of this invention generally arereadily available where the invention is to be practiced, such as acopper smelter. Furthermore, because of the substantial total recoveryof all of the sulfur in the pyrite ores and of the ancillary utilizationof the smelter gas in the process of this invention, which wouldnormally be exhausted to the atmosphere, the incidence of air pollutionby this recovery process is reduced.

SUMMARY OF THE INVENTION The process of the present invention relates tothe recovery of sulfur from pyrrhotite, which is essentially ferroussulfide (FeS). The pyrrhotite used in the present process may be derivedfrom the thermal desulfurization of pyrite (FeS or from pyrrhotite ore.The above desulfurization reaction may be conducted in a flying bedpyrite desulfurizing unit usually operating at a linear gas flow rate at5 feet/second. The present process comprises the utilization of one ormore of the following steps:

a. complexing the ferrous sulfide in aqueous medium with awater-insoluble alkylamino polycarboxylic acid chelating agent to form asolution containing the ferrous chelate of the acid, hydrogen sulfide (H8) and a precipitate of concentrated nonferrous metals;

b. conversion of the evolved H 5 to sulfur by reaction with sulfurdioxide-containing gas (Claus reaction);

c. recovery of the chelating agent by treating the solution of ferrouschelate with an amount of an oxygen-free sulfur dioxide to form thechelating agent and a solution of ferrous bisulfite, under acidicconditions;

(I. Heating the ferrous bisulfite solution to produce sulfur dioxide(oxygen-free) which may be used in (c) above, and a precipitate offerrous sulfite, and pyrolyzing the ferrous sulfite to produceadditional oxygen-free sulfur dioxide and ferrous oxide.

DETAILED DESCRIPTION OF THE INVENTION The pyrrhotite product (FeS) fromthe thermal desulfun'zation of pyrite (FeS containing small quantitiesof nonferrous metals such as copper, molybdenum and silver, is digestedin aqueous medium with an amount of a water insoluble alkylaminopolycarboxylic acid chelating agent, to produce a LII soluble ferrouschelate, liberate hydrogen sulfide and precipitate the nonferrousmetals. The amount of chelating agent is that required to reactstoichiometrically with the ferrous sulfide in the solution. While moreor less than a stoichiometric quantity of chelating agent may be used,it is not preferred. The chelating agent preferably is addedincrementally for best results. This, to obtain complete solubility ofthe ferrous chelate, it is preferred to start with a slurry of theferrous sulfide (pyrrhotite) in water; then the theoretical amount ofchelating agent is added to produce a 5-8 percent suspension of thechelating agent and preferably 7 percent, with thorough mixing. Thechelating agent dissolves only to the extent that it reacts with thepyrrhotite to form the soluble ferrous complex. The reaction may bedemonstrated using EDTA as follows:

The ferrous iron in the ferrous chelate complex is sensitive tooxidation to the ferric state. This oxidation must be avoided becausealthough the chelate may be regenerated from the ferrous complex with S0as will be described in more detail below, it is not possible toregenerate the free chelate from the ferric complex by the action of Forthis reason, it is preferred to conduct the chelation step of thisreaction in the absence of oxygen or an oxidizing agent to avoid anysignificant amount of oxidation. This chelating step may be conducted inthe presence of a reducing agent, such as sodium hydrosulfite or sodiumformaldehyde sulfoxylate, added intermittently during chelatingasrequired to maintain the iron in the ferrous state, as indicated by thedischarge of the red color of the ferric compound.

The complexing of the insoluble pyrrhotite with the insoluble chelatingagent may be conveniently conducted at temperatures above 20 C. up tothe boiling point of the solution and preferably between 70 C. and theboiling point of the aqueous solution. It may be conducted at theboiling temperature with good yields. The reaction is carried out in avessel having a capacity to accommodate the reactants and waternecessary for suspending the reactants and for dissolving the ferrouschelate complex.

While subor superatmospheric pressures may be employed, the chelatingstep preferably is conducted at atmospheric pressures up to about 50p.s.i.g., since this avoids the use of pressure-type equipment.

ble in this invention. Chelating agents such asethylenediaminetetraacetic acid (EDTA ethylenediaminetetrapropionic acid(EDTP) and nitrilotn'acetic acid (NTA) are operable in this invention.EDTA because of its widespread availability, high acidity (high valueacid dissociation constant) and high ferrous chelate stability constant(high value stability constant) is preferred.

Water soluble alkylamino polycarboxylic acid chelating agents such asN-hydroxyethylethylenediaminetriacetic acid (HEDTA) andethylenediaminediproprionic acid (EDDP), because of their solubility,are not recoverable from aqueous solution by precipitation and are notdesired in the operation of this invention. Although they do complexwith ferrous iron and liberate H 5 from FeS, since they would not berecoverable for reuse the cost of the process would become prohibitive.

Other insoluble metal sulfides such as those of copper and molybdenum,CuS and M08 contained in the pyrrhotite are not reactive and do notdissolve in theaqueous solution of ferrous chelate. They are separatedfrom the ferrous iron and concentrated in this step. Silver and gold mayalso be present in the residue as sulfides. These concentrated metalvalues are stockpiled for subsequent recovery by known methods ofroasting, leaching and selective precipitation of the concentratedmetals from the leach solutions. The concentration of these economicallyimportant metals which facilitates their recovery, is an ancillaryadvantage of this process.

The hydrogen sulfide liberated during the chelating step of the presentinventive process is reacted with an SO -containing gas to produceelemental sulfur by the well-known Claus reaction, as described in theEncyclopedia of Chemical Technology, Kirk Othmer, volume 13, page 367.The reaction proceeds as follows:

The reaction is conducted in the presence of a catalyst, such asalumina, bauxite, quartz, and the like. The sulfur dioxide used in thisreaction may be derived from substantially undiluted smelter gas fromthe combustion of pyrite, or from oxidative roasting of sulfide ores. Ifthe smelter gas is allowed to become diluted with air or oxygen as it iscollected from a copper smelter, it is rendered less useful as a sourceof sulfur dioxide in this reaction. Substantially undiluted smelter gascontaining less than 1 to 2 percent air is operable in this step, theless dilute being preferred. Other sources of sulfur dioxide to produceadditional elemental sulfur are available from subsequent reactions asindicated below.

The water-insoluble chelating agent is recovered from the ferrouschelate complex in substantially quantitative yields by treating theaqueous chelate solution, preferably at room temperature, with sulfurdioxide in an amount sufficient to maintain an acid medium having a pHof about 1.0 to 2.0 and preferably about 1.5. The chelating agentprecipitates in substantially pure form in the shape of needles.Temperatures of about 50 C. or lower may be used, but room temperatureis preferred. As the temperature is increased above room temperature,the solubility of the chelating agent increases and the solubility of Sdecreases with corresponding lower recoveries of chelating agent.Therefore, elevated temperatures above room temperature are notpreferred.

in this reaction, as well, for best results, oxygen must besubstantially excluded to prevent oxidation of the ferrous chelate tothe ferric chelate. Overall, the reaction proceeds as follows:

recovered by heating the precipitated ferrous sulfite at about.

ZOO-210C.

The solution, after separating off the ferrous sulfite, may be furthertreated with S0; to insure maximum precipitation of the chelating agent.After filtering off the additional insoluble regenerated chelatingagent, if any, the filtrate may again be boiled to recover S0 andadditional ferrous sulfite as before. This unique cyclic process ofalternatively precipitating the metal sulfite and the complexing agentmay be repeated to increase the accumulative recovery of chelatingagent. The sulfur dioxide regenerated by decomposing ferrous bisulfiteand from pyrolysis of ferrous sulfite, may be divided between the Clausreactor and the chelate regenerator. Additional sulfur dioxide, ifneeded, may be tapped from a source of substantially undiluted smeltergas.

The ferrous oxide residue from the pyrolysis of ferrous sulfite, may beused in metallurgical processes such as in the production of steel, pigiron and similar products.

As is seen from the above description, the process of this invention isself-contained. However, although substantially all of the chelatingagent is recovered and reused, not all is recovered. Additionalchelating agent must be added from time to time to make up for thatwhich is not recovered and slight mechanical losses incurred as aconsequence of repeated use.

The benefits derived from the process of this invention are many.Substantially all of the sulfur in the desulfurized pyrite is recoveredunder comparatively mild conditions. The expensive chelating agent issubstantially all recovered from the ferrous complex and recycled,thereby rendering the process economically feasible. The sulfur dioxideused for regenerating the chelate is recoverable from the ferrousbisulfite product. Additionally, valuable metals such as copper,molybdenum and silver are concentrated in the residue remaining from thepreparation of the ferrous chelate complex above. These residues may becollected and the concentrated metals recovered and separated byconventional known methods.

Referring to the attached chart depicting an overall embodiment usingethylenediaminetetraacetic acid (EDTA), pyrite is thermallydesulfurized+l into ferrous sulfide 2 and sulfur 3. The sulfur isrecovered and the sulfide is treated with EDTA to yield a solubleferrous EDTA chelate 4 and hydrogen sulfide 5. The hydrogen sulfide isreacted with $0 under reaction conditions to yield additional sulfur 3.The ferrous EDTA chelate 4 is treated with an oxygen-free sulfur dioxidegas to precipitate regenerated EDTA 6 which is collected and recycled tostep 2. The ferrous bisulfite solution 7 which forms, is boiled to yieldsulfur dioxide 8 and a precipitate of ferrous sulfite 9. The ferroussulfite 9 is collected and pyrolyzed to yield ferrous oxide 10 andadditional sulfur dioxide 11. The sulfur dioxide products in 11 and 8are collected for use in converting hydrogen sulfide 5 to sulfur or inregenerating the EDTA 4. Additional sulfur dioxide 13 must be providedto satisfy the total requirements for converting hydrogen sulfide tosulfur and regenerating EDTA.

The following example is intended to illustrate a specified embodimentutilizing the present invention and is not intended to limit theinvention thereto.

EXAMPLE 2.2 parts of desulfurized pyrite containing 34.6 percent sulfuris added to a reaction vessel containing parts of water maintained atabout 100 C. While stirring the mixture the stoichiometric amount (7.3parts) of ethylenediaminetetraacetic acid (EDTA) is added. The hydrogensulfide liberated during the complexing of ferrous iron with EDTAamounts is 0.78 parts in 2 hours. This hydrogen sulfide product isreacted with the sulfur dioxide in undiluted smelter gas at 250 C. inthe Claus reaction to produce elemental sulfur. The reaction mixture isthen filtered to collect 0.11 parts of residual solid containingconcentrated metal values such as copper, molybdenum, etc. This residualsolid from each run is stockpiled for subsequent extraction of the metalvalues. The filtrate containing the ferrous EDTA is then treated withsufficient oxygen-free sulfur dioxide in an agitated vessel at 29 C. for4 hours to regenerate the bulk of the EDTA. The pH of the solutionmaintains at about L5 during the reaction. The EDTA precipitated amountsto 7 parts after drying.

The filtrate after filtering off the insoluble EDTA is boiled for about10 minutes yielding a precipitate of ferrous sulfite and additionalsulfur dioxide for recycle.

By this procedure 97.2 percent of EDTA is recovered. Also from theestimated sulfur content of the pyrite, over 99 percent sulfur in pyriteis recovered.

Having thus described our invention, we claim:

1. A process for separating sulfur from a ferrous sulfide ore whichcomprises reacting said ferrous sulfide in aqueous medium with an amountof a water-insoluble alkylamino polycarboxylic acid chelating agentsufficient to effect formation of a water soluble ferrous chelate ofsaid agent and hydrogen sulfide.

2. The process of claim 1 wherein said reaction is effected at atemperature of about 20 C. to the boiling point of the aqueous reactionmixture.

3. The process of claim 1 wherein the chelating agent contains about upto 4 carbon atoms in the alkyl moiety.

4. The process of claim 1 wherein the chelating agent contains about upto 4 carbon atoms in the polycarboxylic acid moiety.

5. The process of claim 1 wherein the chelating agent isethyenediaminetetraacetic acid.

6. The process of claim 1 wherein the chelating agent isnitrilotriacetic acid.

7. The process of claim 1 wherein the hydrogen sulfide product isreacted with sulfur dioxide under conditions sufficient to effectproduction of elemental sulfur.

8. A method of processing a ferrous sulfide ore which comprises thesteps of:

a. reacting said ferrous sulfide in aqueous medium with an amount of awater-insoluble alkylamino polycarboxylic acid chelating agentsufficient to effect formation of a water soluble ferrous chelate andbyproduct hydrogen sulfide; reacting said ferrous chelate solution witha substantially oxygen-free, sulfur dioxide containing medium at a pH ofabout 1 to 2 and in an amount sufficient to regenerate saidwater-insoluble alkylamino polycarboxylic chelating agent and form anaqueous solution of ferrous bisulfite; and

c. separating said chelating agent from the bisulfite solution.

9. The process of claim 8 wherein the aqueous solution of ferrousbisulfite from step (c) is heated to a temperature sufficient to producesulfur dioxide and ferrous sulfite and the ferrous sulfite is heated ata temperature sufficient to desulfurize said ferrous sulfite to produceferrous oxide and sulfur dioxide.

10. The process of claim 9 wherein the sulfur dioxide formed is reactedwith hydrogen sulfide under conditions sufficient to effect productionof elemental sulfur.

11. The process of claim 9' wherein the sulfur dioxide formed is used toregenerate the alkylamino polycarboxylic chelating agent from theferrous complex.

12. The process of claim 8 wherein the regenerated insoluble alkylaminopolycarboxylic chelating agent separated in step (c) is recycled to step(a) for reaction with ferrous sulfide.

13. A process for recovering a water-insoluble alkylamino polycarboxylicacid from an aqueous solution of the soluble ferrous chelate of saidacid which comprises reacting said ferrous chelate solution with asubstantially oxygen-free sulfur dioxide at a pH of about 1 to 2 and inan amount of said sulfur dioxide sufficient to effect conversion of theferrous chelate to the insoluble free acid and ferrous bisulfite insolution and recovering said insoluble acid from said solution.

14. A process as in claim 13 wherein the ferrous bisulfite solution isheated to release sulfur dioxide and precipitate ferrous sulfite andcollecting the sulfur dioxide and ferrous sulfite therefrom.

15. A process as in claim 14 wherein further recovery of water-insolublechelating agent from the resulting solution after removal of the ferroussulfite, is attained by repeating the cycle of treating said solutionwith sulfur dioxide and collecting the precipitated chelating agenttherefrom.

2. The process of claim 1 wherein said reaction is effected at atemperature of about 20* C. to the boiling point of the aqueous reactionmixture.
 3. The process of claim 1 wherein the chelating agent containsabout up to 4 carbon atoms in the alkyl moiety.
 4. The process of claim1 wherein the chelating agent contains about up to 4 carbon atoms in thepolycarboxylic acid moiety.
 5. The process of claim 1 wherein thechelating agent is ethyenediaminetetraacetic acid.
 6. The process ofclaim 1 wherein the chelating agent is nitrilotriacetic acid.
 7. Theprocess of claim 1 wherein the hydrogen sulfide product is reacted withsulfur dioxide under conditions sufficient to effect production ofelemental sulfur.
 8. A method of processing a ferrous sulfide ore whichcomprises the steps of: a. reacting said ferrous sulfide in aqueousmedium with an amount of a water-insoluble alkylamino polycarboxylicacid chelating agent sufficient to effect formation of a water solubleferrous chelate and byproduct hydrogen sulfide; b. reacting said ferrouschelate solution with a substantially oxygen-free, sulfur dioxidecontaining medium at a pH of about 1 to 2 and in an amount sufficient toregenerate said water-insoluble alkylamino polycarboxylic chelatingagent and form an aqueous solution of ferrous bisulfite; and c.separating said chelating agent from the bisulfite solution.
 9. Theprocess of claim 8 wherein the aqueous solution of ferrous bisulfitefrom step (c) is heated to a temperature sufficient to produce sulfurdioxide and ferrous sulfite and the ferrous sulfite is heated at atemperature sufficient to desulfurize said ferrous sulfite to produceferrous oxide and sulfur dioxide.
 10. The process of claim 9 wherein thesulfur dioxide formed is reacted with hydrogen sulfide under conditionssufficient to effect production of elemental sulfur.
 11. The process ofclaim 9 wherein the sulFur dioxide formed is used to regenerate thealkylamino polycarboxylic chelating agent from the ferrous complex. 12.The process of claim 8 wherein the regenerated insoluble alkylaminopolycarboxylic chelating agent separated in step (c) is recycled to step(a) for reaction with ferrous sulfide.
 13. A process for recovering awater-insoluble alkylamino polycarboxylic acid from an aqueous solutionof the soluble ferrous chelate of said acid which comprises reactingsaid ferrous chelate solution with a substantially oxygen-free sulfurdioxide at a pH of about 1 to 2 and in an amount of said sulfur dioxidesufficient to effect conversion of the ferrous chelate to the insolublefree acid and ferrous bisulfite in solution and recovering saidinsoluble acid from said solution.
 14. A process as in claim 13 whereinthe ferrous bisulfite solution is heated to release sulfur dioxide andprecipitate ferrous sulfite and collecting the sulfur dioxide andferrous sulfite therefrom.
 15. A process as in claim 14 wherein furtherrecovery of water-insoluble chelating agent from the resulting solutionafter removal of the ferrous sulfite, is attained by repeating the cycleof treating said solution with sulfur dioxide and collecting theprecipitated chelating agent therefrom.